Fundraisers at the abbey received the donation in a plain brown envelope. Inside the benefactor also said in a note: "Much impressed by the care and consultation which has been put in."
The person's referring to Bath Abbey's Footprint project, which is carrying out essential repairs such as fixing collapsing flooring and using the city's hot springs to create underfloor heating for winter.
The project costs approximately £19.3m in total and this donation is the largest ever sum received by Bath Abbey from private individual, according to The Daily Mail.
Laura Brown, who's leading the Footprint fundraising project, told the newspaper: "It came in an ordinary looking envelope, without pomp or fanfare, but the contents were just astounding.
"We are very grateful for this amazingly generous gift from one remarkable individual."
Bath Abbey now knows who the donor is, however he's insisted he remains anonymous.
The person said: "This generation has a solemn duty to ensure the Abbey is fit and able to carry out its service to the city and the community for the next hundred years."